when illuminated: from a specific direction
uses: the phenomenon of interference to prevent all but a narrow range of wavelengths from passing through. [P6.1]
typically consists: of a thin transparent coating on a glass base (in which case the interference is between beams successively reflected from the back and front surfaces of the coating), or of a thin cavity between two glass plates (in which case the interference is between beams successively reflected from the front and back surfaces of the cavity). [P6.1]
works: by accumulated destructive interference between all wavelengths which are not close to twice the path length between the two reflective surfaces. [P6.1]
Copyright 1997, The Open University